Frequency Modulation (FM) is a method of modulation in which the frequency of a wave is varied in response to a modulating wave. The wave in which frequency is varied is termed the carrier wave and the modulating wave is called the signal. A frequency-modulated electromagnetic wave has a constant amplitude and is much less susceptible to interference from both natural and artificial sources of electromagnetic radiation relative to an amplitude-modulated electromagnetic wave where such sources can cause static. As a result of its favorable performance characteristics, FM is used in a broad range of communication systems such as radio and more recently in personal communication systems (PCS) such as wireless computers and wireless telephony.
An important characteristic of FM based systems is how accurately the original information can be recovered from the frequency-modulated information. The process of recovering original information from frequency-modulated information is typically referred to as demodulation. In an ideal FM system, demodulating a frequency-modulated signal provides only the information that was originally modulated. For example, frequency modulating and subsequently demodulating a 1 KHz signal yields only a 1 KHz signal. In practice however, demodulation produces not only the original signal that was modulated, but also other harmonics. In the prior example, the demodulated information may contain partials at 2 kHz, 3 kHz, 4 kHz, etc. This other information is undesirable because it adversely affects the performance of FM systems. Thus, the power level of the undesirable harmonic distortion is ideally much less than the power of the desired information.
FIG. 1 illustrates a conventional FM delay line demodulator 100 for demodulating a frequency-modulated signal. Demodulator 100 includes a limiter 102, a phase shifter 104, a limiter 106, a mixer 108 and a filter 110. A frequency-modulated input signal, designated as s(t) and having a center frequency of f.sub.IF, is provided to limiter 102. As is well known in the art, a limiter processes an input signal and provides an output signal having a constant, predetermined amplitude, regardless of any amplitude variations that may be present in the input signal. Limiters are used in FM demodulators when circuitry in the demodulator is sensitive to amplitude variations.
Limiter 102 processes input signal s(t) and provides a limited input signal s(t).sub.1, having a constant and predetermined amplitude, to both phase shifter 104 and mixer 108. As is well known in the art and described in more detail hereinafter, mixer 108 is sensitive to variations in signal amplitude and therefore all input signals to mixer 108 must have relatively constant amplitude.
Phase shifter 104 performs a frequency-dependent phase shift of s(t).sub.1 to provide a phase-shifted signal s(t).sub.2. The phase shift is ideally 90 degrees at the center frequency f.sub.IF of s(t). As is described in more detail hereinafter, signal s(t).sub.2 characteristically includes amplitude and frequency distortion added by phase shifter 104. Signal s(t).sub.2 is processed by limiter 106 to generate a phase-shifted signal s(t).sub.3, having a predetermined and constant amplitude, that is provided to mixer 108. Mixer 108 multiplies signal S(t).sub.3 and signal s(t).sub.1 to generate a signal s(t).sub.4 that contains the modulating signal m(t), along with a double frequency term at 2f.sub.IF. Signal S(t).sub.4 is processed by filter 110 to select signal m(t) at the output. Filter 110 is typically a low-pass filter to remove the higher frequency components including noise.
Phase shifter 104 is conventionally implemented as a resistor-capacitor-inductor (RCL) circuit. FIG. 2 illustrates a conventional RCL implementation of phase shifter 104. Phase shifter 104 includes a resistor 200 and a capacitor 202 connected in series between an input voltage (V.sub.IN) and an output voltage (V.sub.OUT). An inductor 204 is connected between V.sub.OUT and a ground 206. The operation of phase shifter 104 as illustrated in FIG. 4 is characterized by the following equations: ##EQU1##
The conventional RCL implementation of phase shifter 104 has several disadvantages compared to an ideal phase shifter. First, when used to process modulated signals having a relatively low center frequency (f.sub.IF) it becomes difficult to obtain a large reactance for "L", as expressed by the numerator. For example, for a Q of 0,1 and a practical resistance of 10K ohms, a 5 mH inductance is needed, which is hard to integrate and is generally impractical for integrated applications. The term "Q" is well understood in the art to mean the quality factor of an RCL circuit. To obtain a relatively high reactance in the numerator of equation (2) above, the value of L must be relatively large, which can be difficult to implement in integrated semiconductor devices because of the large amount of space required to form a large discrete inductor component.
Second, the RCL implementation of phase shifter 104 produces an undesirable amount of amplitude and phase (AM-PM) distortion, requiring the use of limiter 106 and limiting the useful applications for demodulator 100. Because of the characteristically non-linear phase shift provided by phase shifter 104, the demodulated signal m(t) has a relatively low SINAD at low center frequencies such as about 60 KHz. The term "SINAD" is well understood in the art as a figure of merit to express the power (dB) ratio between the ground harmonic and the integration of the higher harmonics (2f.sub.o, 3f.sub.o, etc., where fo is the frequency of m(t)). The amplitude and frequency distortion of phase shifter 104 are explained in more detail with reference to the graphs of FIGS. 3 and 4.
FIG. 3 is a graph 300 illustrating the transfer function 302 of the RCL circuit of phase shifter 104 as a function of frequency. As illustrated by graph 300, the operation of phase shifter 104 over a frequency range 304 (2.DELTA.f) with respect to the center frequency (f.sub.IF) results in a change 306 (.DELTA.H) in amplitude of the phase-shifted output signal s(t).sub.2 of phase shifter 104. Thus, the amplitude of s(t).sub.2 varies (is distorted) over the frequency range of f.sub.IF -.DELTA.f to f.sub.IF +.DELTA.f. Since mixer 108 is very sensitive to changes in amplitude, the phase-shifted output signal s(t).sub.2 of phase shifter 104 must be processed by limiter 106 to provide a signal s(t).sub.3 having a constant amplitude.
FIG. 4 is a graph 400 illustrating the phase change (degrees) over frequency (Hz) of phase shifted-shifted output signal s(t).sub.2 of phase shifter 104, as indicated by line 402. Because of the nonlinear operation of phase shifter 104, as represented by line 402, the demodulated signal m(t) (FIG. 1) has a relatively low SINAD, thereby making demodulator 100 generally unsuitable for high-SINAD applications, such as high quality wireless communications. For comparison purposes, graph 400 includes a line 404 representing an ideal linear phase shift over frequency.
Based on the need to demodulate FM signals and the limitations in the prior approaches, an approach for demodulating FM signals that does not require the use of inductors and that provides a demodulated signal with a SINAD sufficiently high for wireless communication applications is highly desirable.